Average Atomic Mass.   atomic masses reported in periodic table represent: weighted average of masses of all naturally occurring isotopes of an element.

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Presentation transcript:

Average Atomic Mass

  atomic masses reported in periodic table represent: weighted average of masses of all naturally occurring isotopes of an element

Weighted Average:Test Grades   suppose have: 1 test of 0% and 9 tests of 90% What if I said your average was 45%? ( = 90; divide by 2 = 45%)   would you be happy?

NO!   the long way to do this problem: ( ) then  10   average = 81% I bet you like this a lot better!

better way to solve this average!   grades of 90: 9 out of 10 is 90%   grades of 0: 1 out of 10 is 10% 90% test grades are 90 & 10% test grades are 0   so calc average like this… (90%)(90) + (10%)(0) = (0.90 X 90) + (0.10 X 0) = average = 81%

weighted average: better way to calculate average when have numerous items of differing value

Method of Weighted Averages of Isotopes of Atoms ● ● convert % to decimal format (divide by 100) ● ● multiply each isotope’s abundance factor by its atomic mass ● ● find the sum

chlorine’s average atomic mass amu24.230% Isotope amu75.770% Isotope 1 Mass Percent Abundance chlorine has two isotopes

Avg. Atomic Mass of Chlorine = (.75770)( amu) + (.24230) ( amu) (.24230) ( amu) = amu amu = amu convert % to decimals multiply by appropriate mass sum

Avg. Atomic Mass of Si 92.21% of Si has mass of amu 4.70% of Si has mass of amu 3.09% of Si has mass of amu (0.9221)( ) + (0.0470)( ) + (0.0309)( ) = amu